There are many mechanical, electrical, and optical techniques for measuring the relative position of two articles. For example, a mechanical arm may be lengthened or shortened responsive to the movement of the articles. A change in an electrical property such as resistance or capacitance with relative position may be measured. Optical techniques such as light interference measurements or light attenuation in an attenuating medium are used when appropriate.
The available measurement techniques all have drawbacks in various applications. Mechanical measurement techniques impose loads on the system being measured, add substantial weight, are difficult to miniaturize, are difficult to provide with redundancy, and are subject to premature failures. Electrical measurement techniques are often limited to small changes in position, and are therefore not useful when the changes are on the order of many inches, feet, or more. They often require an exposed electrical contact, and usually at least a portion of the electrical measurement apparatus must move with the moving article so that the lead wires must also move. Electrical measurements also suffer from a high sensitivity to the environment of the sensor and to alignment errors. Optical position-measurement techniques typically require a line of sight between the articles, and are extremely sensitive to misalignment.
There is a need for an improved technique for measuring the relative position of two articles which overcomes these drawbacks. The present invention fulfills this need, and further provides related advantages.